The invention relates to internal measurement methods of mobile communication systems, particularly to measurements of signals from adjacent base stations. The invention relates further to a measurement method according to the first claim.
A terminal of a cellular system tries to select a certain base station, within whose coverage area, i.e. cell, it operates. Among other things the cell selection is based on the measurement of the received signal strength (RSSI, Received Signal Strength Indicator) both in the terminal and in the base station. For instance in the GSM system (Global System for Mobile telecommunications) each base station transmits a signal on a certain so called BCCH channel (Broadcast Control CHannel), whereby the terminals measure the strengths of the BCCH signals received by them and conclude on the basis of these signal strengths which cell is the most favourable regarding the radio connection quality. The base stations also transmit to the terminals information about the BCCH frequencies used in the adjacent cells so that the terminals know which frequencies they must monitor in order to find the BCCH transmissions of the adjacent cells.
Regarding the detection of base stations a GSM cellular network at present operates in the following way. For each cell of the GSM network there is defined a list of neighbouring cells which contains the broadcast control channel (BCCH) frequencies of those base stations to which a terminal can next move from the current cell. The mobile station measures the field strengths at the broadcasting channel (BCCH) frequencies of those base stations defined by the list of neighbouring cells, forms the average for each and on the basis of the results it generates a list of the six strongest neighbouring base stations. The aim is to decode the identity codes of the six strongest received neighbours, and a report to the network is made about the levels and identity codes of those base stations for which the identity codes were decoded. The information is used by the network for decisions about change of base station providing service (handover).
In addition to the RSSI measurements the mobile stations measure base station signals also for other purposes, such as in order to determine the time differences of the base station signals. Such time difference measurements (OTD, Observed Time Difference) are used among other things during handover preparation and particularly for mobile station positioning. Essentially OTD means the timing difference between the current serving base station and another base station, as observed by the mobile station. OTD is defined in the ETSI standard ETS 300 912, November 1997, appendix A.1, where the generation of the OTD value is described in more detail.
One method used in mobile communication systems for positioning is the positioning method based on the propagation time of the radio transmission. Positioning based on the propagation time can be made on the basis of the mobile communication network, whereby at least three base stations transmit a radio signal to a mobile station which examines the arrival time differences (OTD, Observed Time Difference) of the signals, or on the basis of the mobile station, whereby the mobile station transmits a radio signal to at least three base stations (BTS, Base Transceiver Station), which examine the observed time difference of arrival (TDOA). On the basis of the arrival times a location service centre (LSC) can obtain at least two hyperbolas, at the intersection of which the mobile station is located. Due to the inaccuracies of the arrival times the hyperbolas are widened to be broad bands having an intersection which defines an area and not a certain point. The positions of the hyperbolas are defined by the positions of the base stations.
The positioning of a mobile station on the basis of OTD measurements requires much time for the measurement. The more accurate measurement result is desired the more time has to be spent for the measurement. In addition to this the RSSI measurements consume available free measurement time. Thus the problem is to obtain sufficiently measurement time for both the RSSI measurements and the OTD measurements. One solution is to use more than one receiver section in the mobile station, but in practice a solution of this type is too expensive. Therefore we discuss below an economically feasible solution where the mobile station has only one receiver section.
The FIGS. 1a to 1d illustrate the prior art situation regarding the RSSI and OTD measurements. FIG. 1a illustrates a common situation where the mobile station communicates with the network using one time slot for transmission TX and one time slot for reception RX. In such a situation the mobile station has good chances to do the RSSI measurements, which in FIG. 1a is illustrated by the rectangle M.
FIG. 1b illustrates a situation where the mobile station in addition to the RSSI measurements also performs OTD measurements. As is seen in FIG. 1b there is still time for this during one frame, but the amount of free time is already clearly smaller than in the situation of FIG. 1a. 
In FIG. 1b and in the other figure the OTD measurement is presented as a measurement with the duration of about 1.5 time slots. For the OTD measurement there is typically used a period which is not quite that long. However, the timing of the OTD measurement is very critical. Even if the measurement would not require the whole period of 1.5 time slots the measurement can be made anywhere during said period. Therefore a period of this length must be reserved for OTD measurements in a TDMA frame.
FIG. 1c shows a situation where a mobile station transmits during two time slots and receives during two time slots. Such a utilisation of multiple time slots is possible for instance according to the HSCSD (High Speed Circuit Switched Data) system which is being developed for the GSM system. There is now less free time because in this case already half of the time slots in a frame are used. However, according to FIG. 1c there is still room for the RSSI measurements.
FIG. 1d illustrates a situation where the mobile station performs OTD measurements in addition to the operations of FIG. 1c. In such a situation the free time of a frame is already consumed, and there are scarcely possibilities for extending the OTD measurements. It is also impossible to make the period between different operations arbitrarily short, such as the period between the end of transmission and the start of OTD measurements, because during said period the mobile station""s synthesiser must be able to switch frequency from the frequency of the previous operation to the frequency of the next operation. This problem can be relieved by using more than one frequency synthesiser, for instance one frequency synthesiser for each operation, whereby the switching of the operating frequency would simply require that the frequency synthesiser output to be used is selected by a switch means. However, a solution of this kind makes the mobile station""s structure more complex and increases the manufacturing costs of the mobile station.
The object of the invention is to reduce the disadvantages of prior art. One object of the invention is also to realise a method for performing RSSI and OTD measurements which leaves more free time in a frame than prior art techniques.
These objects are attained by collecting measurement data at a suitable point during a frame and by performing both RSSI and OTD measurements from the same collected measurement data.
The method according to the invention is characterised in what is presented in the characterising clause of the independent method claim. A mobile station according to the invention is characterised in what is presented in the characterising clause of the claim concerning a mobile station. Other preferred embodiments of the invention are presented in the dependent claims.
In the method according to the invention the RSSI and OTD measurements are made from the same data collected during a certain period of time, whereby the same measurement data is in a way used two times to generate two different sets of measurement results. This saves free time available during a frame, because it is not necessary to provide separate measurement periods for the RSSI and OTD measurements.